SELECT 
  p.product_id, 
  ifnull(
    count(dp.post_id), 
    0
  ) as discussion_amount_posts 
FROM 
  cscart_discussion 
  INNER JOIN cscart_products as p ON (
    cscart_discussion.object_id = p.product_id
  ) 
  INNER JOIN cscart_discussion_posts as dp ON (
    cscart_discussion.thread_id = dp.thread_id 
    AND cscart_discussion.object_type = 'P'
  ) 
WHERE 
  dp.status = 'A' 
  and p.product_id in (
    430644, 430664, 430988, 431073, 430730, 
    431043, 431083, 431135, 430628, 431089, 
    431167, 431459, 339831, 430829, 430842, 
    431063, 431183, 430662, 430844, 431066, 
    431313, 430651, 430869, 430928, 339762, 
    430769, 430880, 430898, 430899, 430884, 
    431035, 339781, 430653, 430684, 430712, 
    430847, 431140, 431142, 430631, 430891, 
    430962, 430626, 431185, 381713, 430893, 
    431093, 430881, 430909, 431031, 339733, 
    339691, 431123, 430797, 430896, 430900, 
    430980, 431197, 431250, 430633, 430806, 
    430873, 430883, 431226, 339600, 430837, 
    431088, 430645, 430685, 430787, 430876, 
    430930, 431023, 431064, 430913, 339488, 
    430768, 430841, 431008, 431078, 431131, 
    339804, 339594, 430676, 430772, 430802, 
    430981, 431119, 430758, 431032, 431121, 
    430760, 430796, 430846, 431113, 431159, 
    430698, 431198, 431356, 381712, 431178
  ) 
GROUP BY 
  p.product_id

Query time 0.00117

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "7.29"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": true,
      "cost_info": {
        "sort_cost": "0.10"
      },
      "nested_loop": [
        {
          "table": {
            "table_name": "dp",
            "access_type": "index",
            "possible_keys": [
              "thread_id",
              "thread_id_2"
            ],
            "key": "thread_id_2",
            "used_key_parts": [
              "thread_id",
              "status"
            ],
            "key_length": "6",
            "rows_examined_per_scan": 19,
            "rows_produced_per_join": 1,
            "filtered": "10.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "4.42",
              "eval_cost": "0.38",
              "prefix_cost": "4.80",
              "data_read_per_join": "851"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ],
            "attached_condition": "(`webmarco`.`dp`.`status` = 'A')"
          }
        },
        {
          "table": {
            "table_name": "cscart_discussion",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY",
              "object_id"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "thread_id"
            ],
            "key_length": "3",
            "ref": [
              "webmarco.dp.thread_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "5.00",
            "cost_info": {
              "read_cost": "1.90",
              "eval_cost": "0.02",
              "prefix_cost": "7.08",
              "data_read_per_join": "2"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type"
            ],
            "attached_condition": "((`webmarco`.`cscart_discussion`.`object_id` in (430644,430664,430988,431073,430730,431043,431083,431135,430628,431089,431167,431459,339831,430829,430842,431063,431183,430662,430844,431066,431313,430651,430869,430928,339762,430769,430880,430898,430899,430884,431035,339781,430653,430684,430712,430847,431140,431142,430631,430891,430962,430626,431185,381713,430893,431093,430881,430909,431031,339733,339691,431123,430797,430896,430900,430980,431197,431250,430633,430806,430873,430883,431226,339600,430837,431088,430645,430685,430787,430876,430930,431023,431064,430913,339488,430768,430841,431008,431078,431131,339804,339594,430676,430772,430802,430981,431119,430758,431032,431121,430760,430796,430846,431113,431159,430698,431198,431356,381712,431178)) and (`webmarco`.`cscart_discussion`.`object_type` = 'P'))"
          }
        },
        {
          "table": {
            "table_name": "p",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY",
              "age_verification",
              "status",
              "idx_parent_product_id",
              "idx_master_product_id"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "product_id"
            ],
            "key_length": "3",
            "ref": [
              "webmarco.cscart_discussion.object_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "0.10",
              "eval_cost": "0.02",
              "prefix_cost": "7.19",
              "data_read_per_join": "760"
            },
            "used_columns": [
              "product_id"
            ]
          }
        }
      ]
    }
  }
}